Discoveries of the chemical elements

The story of the discoveries of the chemical elements is presented here in chronological order. The elements are listed generally in the order in which they were first isolated as the pure element, rather than as a compound (some such as boron were known to be elements decades before they could be isolated from their compounds). The first few predate any written record.

Priestley's work on atmospheric gases resulted
in his preparation of oxygen. As he was a believer in phlogiston, he didn't realise that he had prepared a new
element, and thought that he had managed to prepare air free from
phlogiston ("de-phlogisticated air"). However, he was the
first to isolate oxygen, even if he didn't realise what he had:

The next element was discovered just after the discovery of a new
class of astronomical objects: the new element was named after the
newly discovered asteroid, Ceres. The element was
discovered nearly simultaneously in two laboratories, though it was
later shown that Berzelius and Hisinger's cerium was actually a
mixture of cerium, lanthanum and didymium.

The next element discovered when Mosander showed that the cerium
isolated in 1803 by Berzelius was actually a mixture of cerium,
lanthanum and so-called didymium (which was not actually one element,
and was resolved into two in 1885).

Another spectroscopic discovery, helium was found by astronomers as
an emission line in the spectrum of the sun, hence its name from
the Greek helios meaning sun. It was at first thought to be an
unknown metallic element, and so the name was given the ending -ium
to signify a metal. By the time it had been found on Earth and
discovered to be the lightest of the noble gases, the name was
fixed; by analogy with the other noble gases, the name should have
ended in -on.

In 1871, Mendeleev predicted,
from the gaps in his newly-devised periodic table, that there
should be three as yet undiscovered elements, which he named
eka-boron, eka-aluminium, and eka-silicon. With Mendeleev's
prediction of their existence and approximate chemical properties,
the missing elements were found by French, Scandinavian, and German
chemists, and named for their countries of discovery, as gallium,
scandium, and germanium:

Refrigeration technology advanced considerably during the 19th century, to the point where it was possible to liquefy atmospheric
gases. A curious observation was made: Nitrogen prepared by
chemical means from its compounds had a slightly lower molecular
weight than nitrogen prepared by liquefaction from air. This was
attributed as being due to the presence of a previously unsuspected
gas, christened argon. This gas was the first representative found
of a previously unsuspected new group in the periodic table, first
known as the inert gases, now more commonly known as the noble gases.

Once liquid argon could be prepared in quantity from air, small
amounts of a further three noble gases could be separated from it
by differences in boiling point. These new elements were named from
the Greek words for, respectively, 'new', 'hidden', and 'foreign'.

Another of the noble gases, radon had avoided discovery because its
short radioactive half-life had meant it was present in air in
vanishingly tiny quantities. Once radium was available in
macroscopic quantities, the production of this radioactive noble
gas was readily detected as a product of radium's radioactive
decay.

At this point, all the stable elements existing on earth had been
discovered, and most of the periodic table had been filled. A few
gaps remained amongst the higher mass elements, but there remained
a troublesome gap at element number 43, just below manganese in the
table. The gaps were filled by the synthetic elements.
Walter Noddack and Ida Tacke (later Ida Noddack) also believed to have found Technetium, which they called Masurium (after Masurien, an area in Germany).
They were later proved wrong.

The synthetic elements

The so-called "synthetic" elements are unstable, with
half-lives so "short" relative to the age of the earth that any
atoms of that element that may have been present when the earth
formed have long since completely decayed away. Hence they are
only known on earth as the product of nuclear reactors,
particle accelerators, or in the byproducts from nuclear explosions. The discovery of technetium finally
filled in a puzzling gap in the periodic table, and the
discovery that there were no stable isotopes of technetium
explained its absence on earth: its 4.2 million years
half-life meant that none remained from the time of formation of
the earth.